第10回 TriNet【英語】（Vol.11, No.2, 18-21, 1999/6）Publications

TriNet: Seismic and Strong-motion Recording in Southern California

James Mori (Disaster Prevention Research Institute, Kyoto University)

The deficiencies in our seismic networks for making real contributions toward earthquake hazard reduction were emphasized by the economic losses ($30 billion) in the 1994 Northridge, California earthquake and the large economic losses ($200 billion) and many casualties (over 6000 killed) in the 1995 Kobe, Japan earthquake. Both earthquakes occurred in regions with relatively good seismic monitoring systems, but the instrumental information was lacking in several respects. In the Northridge event, there was extensive damage to a large number of structures, including the well-publicized problem of cracks in the welds of many of the large steel frame buildings (Krawinkler et al.). However, there were relatively few strong-motion recordings to document the actual ground shaking that caused the damage. Kobe had a similar sparseness of strong-motion data, with the additional problem of lack of realtime information that might have facilitated emergency response.

TriNet Goals

The main organizations that record seismic data in southern California (California Institute of Technology (Caltech), U.S. Geological Survey (USGS), and the California Division of Mines and Geology (CDMG) of the Department of Conservation) joined together to form the TriNet project for implementing major improvements in seismic and strong-motion networks. The plan is to build an integrated system that begins to meet the needs of emergency response, engineering concerns in southern California, and the seismological research community. An ambitious instrumental and system development project has started to record and disseminate ground motions from a spatially-dense and robust network of high-quality seismographs in southern California. The project has three main objectives.

1. Rapidly provide ground shaking information in several minutes following damaging earthquakes to increase the effectiveness of emergency response. Point measurements at critical facilities and contour maps of the distributions will be quickly distributed to local emergency response groups so appropriate actions can be taken in heavily impacted areas. Maps of ground acceleration, velocity, and intensity are currently available several minutes after significant earthquakes at http://www.trinet.org/shake.html (Wald et al., 1998; Figure 1).

2. Record ground motion data for improvement of seismic provisions in building codes and seismological research. Weak- and strong-motion data will be used to understand and quantify the levels of shaking during damaging earthquakes. Regulatory decisions on building and retrofit practices need better information on the expected ground motions. The array of wide-dynamic range and broadband instrumentation will also provide valuable recordings of local, regional, and teleseismic waveforms for seismological research. All data will be quickly available from the Southern California Earthquake Center Data Center (http://www.scecdc.scec.org).

Figure 1

3. Develop a prototype Early Warning system. Ground motion information will be rapidly recorded, analyzed, and communicated to test user sites. For Los Angeles it might be possible to provide with 10-40 second warning before the intense shaking from a large San Andreas earthquake.

Instrumentation

The project is divided into two elements with the realtime information emphasis centered at Caltech/USGS in Pasadena and the extensive collection of strong-motion data for engineering purposes centered at CDMG in Sacramento.

By 2002, there will be 670 stations in southern California. Caltech/USGS will be installing broadband and strong-motion sensors with continuous telemetry, expanding the efforts started by the USGS to a total of 200 high dynamic-range sites. CDMG will be installing new sites or upgrading existing instrumentation to provide digital strong motion recording from 400 stations. Many of the strong-motion instruments will be part of the CDMG plan for establishing engineering reference sites throughout the region. Also included are the 70 digital strong-motion sites being installed by USGS National Strong Motion Program (NSMP). The 470 strong-motion sites with be connected with dial-up telephone telemetry, in addition some of the sites will also provide realtime information via continuous telemetry. Current station information can be found on the TriNet homepage, (http://www.trinet.org).

Collaborations

The TriNet Project emphasizes cooperation between federal and state government agencies, university research, and the private sector. It is being built on the infrastructure provided by these groups in Southern California. The CUBE (Caltech-USGS Broadcast of Earthquakes) project started in 1991 has formed a consortium of government agencies and private industry concerned with earthquake hazards in southern California (Kanamori et al., 1991). This group has built support for the project and provided valuable user input into the design of the earthquake information systems.

Lessons for other seismic networks

The experience gained in southern California may provide practical information for other regions that are planning extensive seismic monitoring networks.

- The key to successful cooperation between diverse groups such as earthquake engineers, research seismologists, and emergency planners, is an understanding of the needs and motivations of each group. Conflicting data requirements are often difficult (and sometimes impossible) to reconcile between the various groups when building a multi-purpose system and a successful network requires much coordinated discussion and planning.

- Timeliness should be a priority for getting instrumentation up and running. Bringing parts of the network online over time is better than trying to build the entire system in one step. Network operators have to accommodate social pressures and expectations in the high-profile limelight of earthquake recording.

- Data distribution and archiving are important parts of the seismic system. Well-planned designs for fast data flow, large storage capacity, and easy user accessibility are essential for obtaining the full benefit from the seismic observations.

- Costs are always a major consideration in designing and implementing a network. Purchasing hardware is only part of the cost and should constitute only 50 to 70 percent of the total funds. The costs of installations, software development, and system design are often underestimated in seismic network deployments.